Prior art which is related to this technical field can e.g. be found by the technical specifications TS 36.211 current version: 8.6.0), TS 36.212 (current version 8.6.0) and TS 36.213 (current version: 8.6.0) of the 3GPP.
The following meanings for the abbreviations used in this specification apply:
3GPP: 3rd Generation Partnership Project
ASIC: Application Specific Integrated Circuit
BLER: Block Error Rate
BTS: Base Transceiver Station
CCE: Control Channel Element
CQI: Channel Quality Indicator
DL: Downlink
DSP: Digital Signal Processor
eNB: evolved Node B (eNode B)
FD: Frequency Domain
FDD: Frequency Division Duplex
FEC: Forward Error Coding
GSM: Global System for Mobile Communication
HARQ: Hybrid Automatic Repeat Request
HW: Hardware
LA: Link Adaptation
LTE: Long Term Evolution
OFDMA: Orthogonal Frequency Division Multiple Access
PDCCH: Physical Downlink Control Channel
PDSCH: Physical Downlink Shared Channel
PHICH: Physical Hybrid Indicator Channel
PRB: Physical Resource Block
PS: Packet Scheduler
PUCCH: Physical Uplink Control Channel
PUSCH: Physical Uplink Shared Channel
QoS: Quality of Service
RE: Resource Element
RNTI: Radio Network Temporary Identifier
SC-FDMA: Single Carrier Frequency Division Multiple Access
SRB: Signaling Radio Bearer
SW: Software
TD: Time Domain
TDD: Time Division Duplex
TTI: Transmission Time Interval
UE: User Equipment
UL: Uplink
WCDMA: Wideband Code Division Multiple Access
WiMAX: Worldwide Interoperability for Microwave Access
In recent years, 3GPP's LTE as the upcoming standard is under particular research. The base station of LTE is called eNodeB. LTE will be based on OFDMA in downlink and SC-FDMA in uplink. Both schemes allow the division of the uplink and downlink radio resources in frequency and time, i.e., specific frequency resources will be allocated for certain time duration to the different UE. The access to the uplink and downlink radio resources is controlled by the eNode B, more specifically by the uplink and downlink schedulers that control the allocation of the frequency resources for certain time slots. A time slot is called sub-frame in the LTE specifications.
The concrete resource allocation has to be signaled to the different UE such that these know in which downlink frequencies they can receive the downlink data and on which uplink frequencies they can send their uplink data. The actual data are then transmitted over the PDSCH or the PUSCH, respectively. In addition to this also other information like the used modulation and coding scheme has to be signaled to the UE. This is done via the PDCCH. Here, input information is control information that is sent in uplink over the PUCCH or multiplexed control data that is sent over the PUSCH.
Specifically, the eNB decides before sub-frame n how the UE allocation in downlink and uplink shall be and sends this allocation in the same sub-frame n over the PDCCH to the different UE, while actual uplink transmissions are sent in a later time slot. At the same time, the information whether the previous uplink data block has been received correctly by the eNB or not will be sent over the PHICH. Following this transmission of the PDCCH, the downlink user data is sent to the scheduled UE also in sub-frame n over the allocated frequencies. Then each UE will decode the allocation and the corresponding downlink user data. The uplink user data will then be sent in sub-frame n+4 according to the allocation that has been done by the eNB in sub-frame n (see document TS 36.213). The time delay of 4 sub-frames takes propagation and processing delays into account. While this applies to the FDD mode of LTE, other time delays apply for different TDD configurations.
The scheduling of the user data as well as the allocation of the control resources is ideally done as shown in FIG. 1 illustrating an ideal scheduling/PDCCH management scheme. Specifically, this scheme may use the following steps to decide which UE will be scheduled to the corresponding allocations on the PDCCH and allocate the frequency resources:
Step 1: Downlink and uplink time domain schedulers select a subset of UE that should get downlink and uplink resources to transmit their data. The decision will be based on quality of service aspects, data availability, pending HARQ retransmissions etc. Therefore, this information needs to be available at this point of time.Step 2: Allocation of signaling resources on PDCCH by the PDCCH manager. This step evaluates the needed modulation (depending on e.g. LTE release) and coding format, transmission power and the allocation of the corresponding signaling resources. This allocation will also take into account the QoS and the HARQ status of the corresponding UE. Since the UE will only be able to decode this signaling information for a limited search range of the PDCCH channel, there is a risk of blocking of some of the UEs that should get allocated according to the TD scheduler. Those will then be postponed to the next available scheduling instant. However, the UEs that have uplink HARQ retransmissions should preferably get resources, since LTE uses a synchronous uplink HARQ mode, and a missed scheduled retransmission will cause an additional retransmission delay of 8 ms (for FDD mode). Therefore, these UL retransmissions will be allocated with higher priority. This steps needs to be handled before the final allocation of resources on the PDSCH/PUSCH.Step 3: Downlink and uplink frequency domain schedulers allocate the frequency resources for the different UE on PDSCH and PUSCH, respectively.
As a further aspect, according to document TS 36.211, in LTE the PDCCH carries UE-specific control information to each scheduled UE both in uplink, i.e. UL grant, and downlink direction, i.e. DL scheduling assignment, each TTI, i.e. time slot. The PDCCH is mapped onto a set of RE or similarly onto one or more control channel elements (CCEs), where 1 CCE=36 RE in up to the first three OFDM symbols of a TTI (2-4 OFDM symbols are permitted for a system bandwidth of 1.4 MHz). Given a certain system bandwidth and a defined number of OFDM symbols to be used for PDCCH transmissions, the total amount of available CCE is known. An example number of CCE available for dynamic scheduling for a 10 MHz configuration would be 10, 27, and 43 CCE for 1, 2, and 3 OFDM symbols for control, respectively. While the packet scheduler decisions are taken independently for UL and DL as they assign UL and DL resources, respectively, the allocation of the common control resources by the PDCCH scheduler needs to be performed jointly for UL and DL. The number of allocated CCE, i.e. the CCE aggregation level, for different UE depends on the dynamically estimated channel conditions of the target UE, hence a link adaptation algorithm is used on PDCCH to select the dynamic aggregation level per user. At present, it is considered that the packet scheduler will be able to perform the decisions on the scheduling of different users jointly (see FIG. 1).
Specifically, an eNB as the LTE base station has not only to signal the corresponding allocation for UL and DL to all scheduled UE on the PDCCH per TTI. In addition, also broadcast, paging and other common signaling is to be transmitted. In order to comply with these tasks, the PDCCH is partitioned into a common search space (CSS) and a UE specific search space (USS). Every active UE in the cell listens to the PDCCH. In this context, an active UE is covering the UE that are in active mode, while at the same time has a discontinuous reception (DRX) pattern that allows for decoding of the PDCCH. Though, a UE listens only on specific search positions according to its hashing function, which relies on either the cell-specific or semi-persistent RNTI, and to a sub-frame number and the aggregation selected for the message. An aggregation defines the code-rate selected for the message, which is derived from CQI/radio quality measurements such that typically a target of 1% BLER is maintained. Unfortunately, the higher the aggregation, the lower is the number of potential search positions on the PDCCH. There are aggregations 1, 2, 4 and 8 possible with six potential search positions, six potential search positions, two potential search positions, and two potential search positions, respectively, on PDCCH available.
By referring again to FIG. 1, the UL and DL TD PS output independent sub-sets of prioritized UL and DL users, respectively. Before UL and DL frequency domain PS can define the final list of scheduled UE and independently perform the PRB allocation, the downlink control channel resource scheduler needs to perform the following:
1) Determining the Number of Needed OFDM Symbols (1-3) for Control
Several alternatives are possible, e.g., from semi-static allocation to a dynamic PDCCH symbol allocation to optimize the trade-off between data transmitted and control overhead. As output of this step the total amount of CCE available for dynamic scheduling is known.
2) Share the CCE Dynamically Between UL and DL Scheduled UE
This step may potentially limit the number of UEs who can be scheduled in the next TTI due to control channel resource limitations, but should target to share in a relative fair manner the control resources for the two link directions. In this connection, also certain QoS aspects need to be considered, i.e. if one direction has more QoS traffic than the other one. In such a case, it potentially needs more resources. In addition, the UL typically needs more PDCCH resources, since more UE need to be scheduled per sub-frame than in DL due to the fact that the UE power is much smaller than the eNode B power. Furthermore, for certain TDD configurations, some subframes will be DL only, while others in contrast will potentially favor UL allocations more, as the next UL scheduling opportunity happens much later.
3) Map Each UE to the Proper Aggregation and Power Boosting Level, while Still Satisfying the BLER Target for the PDCCH
This step should be performed applying hashing-functions as defined in the respective standard specifications.
In the steps above where the eNB scheduler have to assign CCE resources for a given scheduled UE, the eNB will have to take the UE's search space into consideration. As described above, the search space defines a set of search positions that the UE will investigate for scheduling information intended for it. FIG. 2 shows a table describing the positions of the search space as defined by Table 9.1.1-1 of document TS 36.213.
Specifically, according to document TS 36.213, a control region consists of a set of CCEs, numbered from 0 to NCCE,k−1, where NCCE,k is the total number of CCEs in the control region of subframe k. The UE shall monitor a set of PDCCH candidates for control information in every non-DRX sub-frame, where monitoring implies attempting to decode each of the PDCCHs in the set according to all the monitored DCI formats. The set of PDCCH candidates to monitor are defined in terms of search spaces, where a search space Sk(L) at aggregation level Lε{1,2,4,8} is defined by a set of PDCCH candidates. The CCEs corresponding to PDCCH candidate m of the search space Sk(L) are given by L·{(Yk+m)mod └NCCE,k/L┘}+i, where Yk is defined below, i=0, . . . , L−1 and m=0, . . . , M(L)−1. M(L) is the number of PDCCH candidates to monitor in the given search space. The UE shall monitor one common search space at each of the aggregation levels 4 and 8 and one UE-specific search space at each of the aggregation levels 1, 2, 4, 8. The common and UE-specific search spaces may overlap. The aggregation levels defining the search spaces are listed in Table 9.1.1-1 of document TS 36.213, shown in FIG. 2. The DCI formats that the UE shall monitor depend on the configured transmission mode. For the common search spaces, Yk is set to 0 for the two aggregation levels L=4 and L=8. For the UE-specific search space Sk(L) at aggregation level L, the variable Yk is defined by Yk=(A·k-1)mod D, where Y−1=nRNTI≠0, A=39827, D=65537 and k=└ns/2┘, ns is the slot number within a radio frame. The RNTI value used for nRNTI is defined differently for downlink and uplink.
The UE specific search space defines search positions that will be scattered at ‘random’ positions, while the common search space is well-defined and common for all UE monitoring the PDCCH.
FIG. 3 illustrates the PDCCH common search space (boxes filled with horizontal lines) which is common for all UE monitoring the PDCCH and an example of a user specific search space (boxes with diagonal lines). From top to bottom, the respective four rows represent aggregation level 8, 4, 2 and 1, respectively.